1. Field of the Invention
This invention relates generally to the field of electronic hearing aids for improving the hearing of a human subject and, in particular, to a method for optimizing the gain of a hearing aid and preventing feedback instabilities.
2. Prior Art
Hearing aids are active electronic devices which detect sound with a miniature microphone, amplify and filter the sound with an electronic circuit, and deliver the amplified sound via a miniature loudspeaker called a "receiver." The amplified sound output of the receiver may be detected by the microphone, amplified by the circuit, and delivered again to the receiver, causing the process to be repeated. Thus, the sound energy is propagated in a "closed loop" from the hearing aid input, to the output, and back to the input. The path of the sound energy from the receiver to the microphone may either be acoustical or mechanical. Sound energy at some frequencies will add in phase as it travels around the closed loop. At these frequencies the energy builds up very rapidly, which saturates the hearing aid transducers and circuit and causes audible feedback, distortion, and general instability of the system.
These effects of unstable feedback render the hearing aid useless and produce an annoying howl or squealing sound that draws unwanted attention to the hearing aid user. The conditions which cause unstable feedback are well understood, and the means of preventing it are straightforward. The amplification or gain of the hearing aid can be reduced, or the tightness of the hearing aid fit in the ear canal can be increased. (The fit of the hearing aid in the ear canal and the use of small holes in the hearing aid shell for venting air pressure both determine the acoustic attenuation of the feedback path from the receiver to the microphone.) Both of these preventative measures have undesirable side effects, however. Too much reduction of hearing aid gain, or reductions at the wrong frequencies, may mean that the user will be unable to hear certain sounds. Likewise, tightening the fit of the hearing aid in the ear canal or blocking the vent may make the hearing aid uncomfortable to wear.
Heretofore, no simple and accurate methods have been available to the hearing aid dispenser for determining when unstable feedback will occur for a particular user and hearing aid, or what frequencies are causing the unstable feedback.
There are two principal prior art methods of preventing unstable feedback in hearing aids. The first method employs trial and error to adjust the hearing aid response. The dispenser observes that unstable feedback occurs with a particular hearing aid fitting and then either reduces the hearing aid gain or tightens the fit of the hearing aid in the ear canal. This process is repeated until unstable feedback no longer occurs. With such a trial and error process, dispensers are likely to make too large a gain adjustment. The consequences for the hearing aid user are too little gain and reduced hearing aid benefit.
The second method requires the use of a two-channel spectrum analyzer and the ability to break into the hearing aid circuit (e.g. at the output of the microphone ). This method is illustrated in FIG. 1. Hearing aid 10 comprises microphone 12, preamplifier 14, frequency dependent amplifier 16 and receiver (loudspeaker) 18. The hearing aid 10 is fitted in the user's ear canal. A test signal is generated by a spectrum analyzer 20, usually a wide bandwidth, flat spectrum audio noise signal, and provided as an input to the hearing aid amplifier in place of the microphone signal. This test signal is also delivered to the channel one input of the two-channel analyzer. The test signal is amplified by the hearing aid amplifier 16 and delivered to the receiver 18. The receiver output travels from B to A via the feedback path and is detected by the microphone 12. The output of the microphone preamplifier 14 is routed to the channel two input of the analyzer, and the open loop transfer function of the hearing aid in the ear canal is computed from the two channel inputs. From this transfer function, frequencies are identified at which the magnitude response is greater than or equal to unity and at which the wrapped phase response passes through 0.degree.. In the closed loop system, which represents the actual hearing aid and feedback path, instability will occur at these frequencies because feedback will add in phase and cause sound energy to build up and saturate the system. The open loop magnitude response is adjusted by reducing the gain of the hearing aid at the unstable feedback frequencies so that the magnitude response is slightly less than unity.
There are substantial disadvantages to both of the above-disclosed prior art methods. This first method is a trial and error method that is prone to over-adjustments and mis-adjustments of hearing aid gain. This method is applied only after unstable feedback has been observed to occur. The dispenser does not know at which frequencies to make the gain adjustments, nor how large the adjustments should be. Over-adjustment of hearing aid gain to prevent unstable feedback can reduce the benefit of the hearing aid.
The disadvantages of the second method have mainly to do with its practicality. The method requires the hearing aid to be designed so that the internal signal path from the microphone to the amplifier can be interrupted for measurements with a two-channel spectrum analyzer. Special connectors must be mounted on the hearing aid to allow access to the signals. No known commercial hearing aids are designed in this fashion.
Feedback is a practical problem that can limit the performance and benefit of any hearing aid. Prior to the current invention, there have been no available methods for accurately predicting the frequencies and gains that will cause unstable feedback for a particular hearing aid response in a particular individual's ear. New digital hearing aid technologies are especially susceptible to feedback problems because of the additional delays introduced by digital processing in the hearing aid circuit.